As the data areal density in hard disk drive (HDD) writing increases, write heads and media bits are both required to be made in smaller sizes. However, as the write head size shrinks, its writability degrades. To improve writability, new technology is being developed that assists writing to a media bit. One approach that is currently being investigated is microwave assisted magnetic recording (MAMR), which is described by J-G. Zhu et al. in “Microwave Assisted Magnetic Recording”, IEEE Trans. Magn., vol. 44, pp. 125-131 (2008). Although MAMR has been in development for a number of years, it is not shown enough promise to be introduced into any products yet because of several technical problems. One problem is a fringing growth when the spin torque oscillator (STO) bias is turned on. Thus, when a STO layer is inserted into the write gap, and magnetization therein is flipped to be anti-parallel to the magnetic field in the gap, the reluctance in the gap is increased, and write field as well as the return field to the trailing shield are boosted. However, fringing will grow as the write field increases, and dramatically decrease the writer tracks per inch (TPI) capability.
Spin transfer (spin torque) devices are based on a spin-transfer effect that arises from the spin dependent electron transport properties of ferromagnetic-non-magnetic spacer-ferromagnetic multilayers. When a spin-polarized current passes through a magnetic multilayer in a CPP (current perpendicular to plane) configuration, the magnetic moment of electrons incident on a ferromagnetic layer interacts with magnetic moments of the ferromagnetic layer near the interface between the ferromagnetic and non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the ferromagnetic layer. As a result, spin-polarized current can switch the magnetization direction of the ferromagnetic layer if the current density is sufficiently high.
In a MAMR writer, the main pole generates a large local magnetic field to change the magnetization direction of the medium in proximity to the writer. By switching the direction of the field using a switching current that drives the writer, one can write a plurality of media bits on a magnetic recording medium. In MAMR, a STO has a ferromagnetic layer called a field generation layer (FGL) wherein a magnetization is driven into a precessional state when spin torque is applied. As a result, a RF field is generated on the magnetic medium and induces a precessional state in one or more bit magnetizations, which lowers the coercivity therein and reduces the MP field and switching current necessary for a write process.
Magnetic flux in the main pole proceeds through the ABS and into a medium bit layer and soft underlayer (SUL). In some common designs, the flux returns to the write head through a trailing side loop comprised of a trailing shield structure, and through a leading side loop that includes a leading shield and back gap connection. There is also a gap field flux that exits the main pole through the write gap, side gaps, and leading gap, and is not directly responsible for writing. It is desirable to reduce the flux through the gap surrounding the MP in order to improve track per inch (TPI) and bits per inch (BPI) capability. Current MAMR writer designs have a STO at the ABS, but STO reliability is a concern because of wear that results from touchdown during multiple write processes. Thus, an improved STO design is needed that improves reliability compared with existing STO devices while maintaining the capacity to increase reluctance in the gap around the MP, and with some applied current densities, generate a RF field on a bit magnetization in a magnetic medium to enable a lower write field.